1. Field of the Invention
The present invention generally relates to a fault detecting apparatus for detecting abnormality or fault in an exhaust gas recirculation control system of an internal combustion engine (hereinafter also referred to simply as the engine). In particular, the invention is concerned with a fault detecting apparatus which is improved in respect to accuracy and reliability by suppressing influences of various factors or parameters involved in fault detection processes.
2. Description of Related Art
Heretofore, in the field of the engine control systems for the automobiles or motor vehicles, the exhaust gas recirculation control techniques for feeding back or recirculating a part of exhaust gas to the engine for thereby lowering the combustion temperature for the purpose of suppressing NO.sub.x -components contained in the engine exhaust gas are widely known. For having better understanding of the background techniques of the present invention, description will first be made in some detail of conventional exhaust gas recirculation control systems.
FIG. 21 is a block diagram for illustrating schematically a general arrangement of an internal combustion engine system equipped with an exhaust gas recirculation control system known heretofore.
Referring to FIG. 21, the internal combustion engine system is comprised of an engine body 1 having a plurality of cylinders, an air cleaner 2 for purifying intake air to be introduced into the engine, an intake pipe 3 for feeding the air introduced through the air cleaner 2 to the engine, an intake 4 for connecting the intake pipe 3 to the plurality of cylinders of the engine 1, a fuel injector 5 for injecting fuel into the PG,3 engine cylinders, a pressure sensor 6 for detecting a intake pressure Pb within the intake 4 or within the intake pipe 3 at a position located in the vicinity of the intake 4 (this pressure will be referred to as the intake pressure), a throttle valve 7 disposed within the intake pipe 3 for controlling an intake air flow, a throttle position sensor 8 for detecting an opening degree .theta. of the throttle valve 7, and a linear-solenoid type bypass air flow rate control means 9 for controlling an air flow rate which bypasses the throttle valve 7 via a pipe connected across the throttle valve 7 in parallel to the intake pipe 3.
An exhaust gas recirculation pipe (hereinafter also referred to as the EGR pipe) 10 is provided for feeding back or recirculating a part of the exhaust gas discharged from the engine 1 to the intake pipe 3. An exhaust gas recirculation valve (hereinafter also referred to as the EGR control valve) 11 of a vacuum-motor-driven type is installed in the EGR pipe 10 for controlling the flow rate of the exhaust gas flowing through the EGR pipe 10. Opening and closing of the EGR control valve 11 is controlled by a three-way solenoid valve device (hereinafter referred to as the EGR solenoid device) 12 which has inlet ports communicated to the intake pipe 3 and the atmosphere, respectively, and an outlet port communicated to the EGR control valve 11. The EGR control valve 11 and the EGR solenoid device 12 cooperate to constitute an exhaust gas recirculation flow control means for adjusting the exhaust gas recirculation flow rate in dependence on the operation states of the engine 1 under the control of an electronic control unit 22, which will be made apparent later on.
An ignition coil 13 serves for generating a high voltage required for combustion of air/fuel mixture gas within the individual cylinders of the engine 1. Provided in association with the ignition coil 13 is a firing or ignitor circuit 14 for interrupting a primary current of the ignition coil 13 to thereby generate a spark for triggering combustion of the air/fuel mixture. The exhaust gas resulting from the combustion within the engine cylinders is discharged through an exhaust pipe 15. A catalytic converter 16 for purifying the exhaust gas is installed in the exhaust pipe 15 at a position downstream of a location from which the EGR pipe 10 is branched.
An ignition signal Q generated by the ignition coil 13 adapted to be driven by the ignitor 14 has a frequency which corresponds to the rotation speed (rpm) of the engine 1 and thus can be utilized as a sensor signal indicative of the rotation speed or number (rpm) of the engine 1. Further, as other engine operation state sensor means, there are provided a water temperature sensor 17 for detecting a temperature T of the cooling water of the engine 1 and an idle switch 18 for detecting whether or not the throttle valve 7 is in the fully closed state (i.e., the state in which the opening degree of the throttle valve is zero), to thereby generate an idle signal I when the throttle valve 7 is in the fully closed state. An air-conditioner on/off switch 19 is provided for generating an air-conditioner power-on signal A serving as an on/off command for an air conditioner (not shown) which represents a typical one of the engine loads. An air-conditioner controller 19A is adapted to control the air conditioner in accordance with an air-conditioner control signal D generated by the electronic control unit 22 in response to the air-conditioner power-on signal A by taking into account the operation state of the engine.
The pressure sensor 6, the throttle position sensor 8, the ignition coil 13, the water temperature sensor 17, the idle switch 18, the air-conditioner on/off switch 19 and others cooperate to constitute a sensor means which provides information concerning the operation states of the engine 1. An ignition key switch 21 is closed upon starting of the engine operation for supplying an electric power to various electric/electronic units and devices of the motor vehicle from an onboard battery 20.
The electronic control unit 22 mentioned previously is constituted by a computer system. The electronic control unit 22 which is put into operation upon reception of an electric power from the battery 20 is designed to fetch from a variety of sensor means mentioned above the engine operation state information such as those typified by the throttle opening degree .theta., the idle signal I, the intake manifold pressure Pb, the cooling water temperature T, the ignition signal Q (i.e., engine speed (rpm) signal), the air-conditioner power-on signal A and others to thereby control the fuel injection amount, the exhaust gas recirculation flow rate and the bypass air flow rate, respectively, in addition to the control of the air-conditioner.
More specifically, the electronic control unit 22 includes a fuel control means, an exhaust gas recirculation control means, an exhaust gas recirculation system fault decision means and others, whereby a fuel injection control signal J for the fuel injector 5, an EGR control signal for the EGR solenoid device 12, a bypass control signal B for the bypass air flow rate control means 9 and the air-conditioner control signal D for the air-conditioner controller 19A are outputted from the electronic control unit 22.
FIG. 22 is a block diagram showing in detail a functional configuration of the electronic control unit 22 of FIG. 21. Referring to FIG. 22, a microcomputer 100 includes a CPU (Central Processing Unit) 200 for generating the various control signals J, C, B and D mentioned above on the basis of the aforementioned engine operation state information Q, Pb, .theta., T, I and A in accordance with predetermined programs, a free-running counter 201 for measuring a period of rotation cycle of the engine 1, a timer 202 for measuring timings and temporal durations for the various controls, an analogue-to-digital converter (hereinafter referred to as the A/D converter) 203 for converting analogue input signals into digital signals, an input port 204, a RAM (Random Access Memory) 205 used as a work memory, a ROM (Read-Only Memory) 206 for storing various processing programs, an output port 207 for outputting the fuel injection control signal J, the exhaust gas recirculation control signal C, the bypass control signal B and the air-conditioner control signal D, and a common bus 208 for interconnecting the CPU 200 with the various components 201 to 207 mentioned above.
The electronic control unit 22 further includes a first input interface circuit 101 for shaping the ignition signal Q for the ignition coil 13 to thereby generate an interrupt signal to be inputted to the microcomputer 100. Thus, upon every generation of the ignition signal Q as the interrupt signal, the CPU 200 incorporated in the microcomputer 100 reads the count value from the counter 201 to calculate the rotation period of the engine 1 on the basis of a difference between the count values read out at the current time point and at a preceding time point, respectively. The engine rotation period thus determined is then stored in the RAM 205.
The electronic control unit 22 includes a second input interface circuit 102 which serves for fetching the intake pressure Pb, the throttle opening degree signal .theta. and the cooling water temperature T, respectively, from the pressure sensor 6, the throttle position sensor 8 and the water temperature sensor 17. These sensor signals are inputted to the A/D converter 203.
Further, the electronic control unit 22 includes a third input interface circuit 103 through which the idle signal I and the air-conditioner power-on signal A are fetched from the idle switch 18 and the air-conditioner on/off switch 19, respectively, to be supplied to the input port 204.
On the other hand, an output interface circuit 104 of the microcomputer 100 serves to receive the various control signals J, C, B and D from the output port 207 to thereby output these control signals to the fuel injector 5, the EGR solenoid device 12, the bypass air flow rate control means 9 and the air-conditioner controller 19A, respectively, after amplification and shaping of the control signals.
Next, the exhaust gas recirculation control operation of the conventional control system will be described by reference to FIGS. 21 and 22.
When the EGR solenoid device 12 is electrically energized in response to the EGR control signal C, a negative pressure is applied to a negative pressure chamber of the EGR control valve 11, as a result of which the EGR control valve 11 is opened, whereby a part of the engine exhaust gas is recirculated to be introduced into the engine 1.
On the other hand, when the EGR solenoid device 12 is turned off in response to the EGR control signal C, the atmospheric pressure is applied to the negative pressure chamber of the EGR control valve 11, which will result in closing of the EGR control valve 11 and hence inhibition of recirculation of the exhaust gas into the engine 1. In this manner, the EGR solenoid device 12 controls introduction of the exhaust gas to the engine 1 in response to the EGR control signal C.
The bypass control signal B for the bypass air flow rate control means 9 which may be constituted by an ISC solenoid valve device is supplied in the form of a pulse signal having a duty ratio which is controllable. Thus, when the duty ratio of the bypass control signal B is increased, the current flowing the bypass air flow rate control means 9 increases correspondingly. As a result of this, the flow area of the ISC solenoid valve device is increased, whereby the cross sectional area of the air passage bypassing the throttle valve 7 increases. In this manner, the bypass air flow rate can be controlled.
The engine load driving means incorporated in the electronic control unit 22 generates the air-conditioner control signal D for actuating the air conditioner when the air-conditioner power-on signal A indicates the command "ON" and when the engine operation state satisfies the condition which permits the air conditioner to be put into operation. On the other hand, when the air-conditioner power-on signal A commands the turn-off of the air conditioner, the air-conditioner control signal D for deenergizing the air conditioner is generated. In this way, the air conditioner is controlled with preference being put on the engine operation state with a view to protecting the engine against application of an excessively large load.
Next, description will turn to the operation of a hitherto known fault or abnormality detecting apparatus for the exhaust gas recirculation control system implemented in the structure described above by reference to FIGS. 21 and 22 on the assumption, by way of example only, that the fault detection is carried out in the deceleration state of the engine. FIG. 23 is a flow chart for illustrating a conventional fault detection processing executed by the CPU 200 incorporated in the electronic control unit 22 for detecting occurrence of a fault or abnormality in the exhaust gas recirculation control system.
At first, in a step S101, it is checked from an engine rotation number Ne (rpm) determined previously on the basis of the ignition signal Q through a proper processing routine (not shown) and the idle signal I outputted from the idle switch 18 whether the engine rotation number Ne is higher than a predetermined value and whether the throttle valve 7 is in the fully closed state (i.e., the idle signal I is at the on-level). When both the conditions mentioned above are satisfied, it is then decided that the motor vehicle is in the state of deceleration (i.e., the conditions prerequisite for making the decision concerning occurrence of a fault or abnormality in the exhaust gas recirculation control system are met or satisfied).
When it is decided in the above-mentioned step S101 that the motor vehicle is not in the deceleration state (i.e., when the decision step S101 results in negation "NO"), the fault detection processing illustrated in FIG. 23 is terminated, as represented by RETURN. In contrast, when the decision step S101 results in affirmation "YES", indicating that the motor vehicle is in the deceleration state, the processing proceeds to steps 102 et seq.
In the step S102, the EGR solenoid device 12 is electrically deenergized with the exhaust gas recirculation being invalidated or set to the EGR-off state, which is then followed by execution of the step S103 where the intake manifold pressure Pb in the EGR-off state is stored as a value PbOFF (hereinafter also referred to as the EGR-off intake pressure value). Parenthetically, it should be mentioned that in the deceleration state of the motor vehicle, the exhaust gas recirculation is normally invalidated in the initial state. Accordingly, it is unnecessary to turn off forcibly or positively the EGR solenoid device 12.
Subsequently, in a step S104, the EGR solenoid device 12 is forcibly turned on to open the EGR valve 11 for thereby validating the exhaust gas recirculation (i.e., set up the EGR-on state). In a next step S105, the intake manifold pressure Pb is fetched in the EGR-on state to be stored as a value PbON (hereinafter also referred to as the EGR-on intake pressure value).
In this conjunction, it will readily be understood that there will make appearance a difference between the EGR-off intake pressure value PbOFF and the EGR-on intake manifold pressure value PbON so long as the exhaust gas recirculation control system is operating normally without suffering any fault or abnormality. Accordingly, in a step S106, a pressure difference .DELTA.P between the EGR-on intake manifold pressure value PbON and the EGR-off intake manifold pressure value PbOFF is arithmetically determined in accordance with: EQU .DELTA.P=PbON-PbOFF
In succession, in a step S107, it is decided whether or not the intake manifold pressure difference .DELTA.P mentioned above is greater than a preset decision reference value FAIL (representing a lower limit value of the intake manifold pressure difference .DELTA.P in the normal state of the exhaust gas recirculation). When the result of the decision in the step S107 is affirmative or "YES" (i.e., when .DELTA.P.gtoreq.FAIL), this means that the intake manifold pressure difference .DELTA.P has a normal value (indicating the normal exhaust gas recirculation state). Accordingly, in a step S108, it is decided that the exhaust gas recirculation control system operates normally without suffering any abnormality.
On the other hand, when the decision result of the step S107 indicates that .DELTA.P&lt;FAIL (i.e., when the step S107 results in negation "NO"), this means that the intake manifold pressure difference .DELTA.P does not reach the lower limit value of the normal pressure difference (i.e., exhaust gas recirculation is not normally carried out). Accordingly, decision is made in a step S109 that the exhaust gas recirculation control system suffers abnormality.
Incidentally, when the engine deceleration state is decided in the step S101 mentioned above, the intake manifold pressure Pb can first be fetched in the EGR-off state (steps S102 and S103) without manipulating the EGR control valve 11 because the latter must normally be in the fully closed state when the motor vehicle is in the deceleration state. Subsequently, the EGR control valve 11 is forcibly set to the fully opened state for validating the exhaust gas recirculation by energizing the EGR solenoid device 12, whereon the intake manifold pressure value PbON is fetched in the EGR-on state (steps S104 and S105). However, because it is undesirable to terminate this routine in the state where the exhaust gas recirculation is effectuated, the processing is terminated in practice after the EGR control valve 11 is again set to the fully closed state (i.e., after the EGR-off state is regained).
In this conjunction, it should be mentioned that when the EGR control valve 11 is in the fully closed state (i.e, in the EGR-off state), the intake manifold pressure Pb is normally on the order of 260 mmHg, while when the exhaust gas is forcibly introduced into the engine with the EGR control valve 11 being fully opened (i.e., in the EGR-on state), the intake manifold pressure Pb amounts to ca. 460 mmHg because of a steep increase in the flow rate of the intake air introduced into the engine 1, although it depends on the specifications of the engine and the operation states thereof.
Thus, the intake manifold pressure difference .DELTA.P calculated in the step S106 will assume a value of about 200 (=460-260) mmHg. Under the circumstances, the preset decision value FAIL used as the reference value in the comparison at the step S107 should preferably be set at, for example, 100 mmHg so that it can definitely be discriminated from the normal value (200 mmHg) of the intake manifold pressure difference .DELTA.P.
As can be seen from the foregoing, the fault detection for the exhaust gas recirculation control system can be realized by making use of the fact that difference in the quantity of the intake air (i.e., difference between the fresh intake air and a sum of the recirculated exhaust gas and the fresh intake air) is reflected onto the intake manifold pressure Pb. Of course, occurrence of abnormality in the exhaust gas recirculation control system as detected in this way may be informed to the driver by turning on, for example, an alarm lamp or the like device through an appropriate processing routine (not shown).
Next, assuming that the engine is in the stable state, a fault detection processing for the exhaust gas recirculation control system known heretofore will be described by reference to a flow chart of FIG. 24.
Referring to the figure, in a step S211, it is checked on the basis of the engine rotation number Ne (rpm) and the throttle opening degree .theta., whether deviations (changes) in the engine rotation number Ne and the opening degree .theta., respectively, are smaller than or equal to respective preset reference values, to thereby decide whether or not the engine or the motor vehicle is in the stable state (i.e., whether the condition prerequisite to the decision of occurrence of a fault in the exhaust gas recirculation control system in the stable state is satisfied or not).
When the decision step S201 results in negation "NO", indicating that the motor vehicle is not in the stable state, the fault detection processing routine illustrated in FIG. 24 is terminated (RETURN). On the contrary, when the answer of the decision step S201 is affirmative "YES", indicating the stable operation state of the engine, the processing proceeds to steps S212 et seq. (corresponding to those S102 et seq. in FIG. 23).
At first, in the step S212, the EGR solenoid device 12 is activated to validate the exhaust gas recirculation, whereupon the intake manifold pressure Pb in the EGR validated state is stored as the EGR-on intake manifold pressure value PbON. Parenthetically, it should be mentioned that since the exhaust gas recirculation is validated already in the initial state when the motor vehicle is in the stable state, there exists no necessity for positively actuating the EGR solenoid device 12 for controlling the EGR control valve.
Subsequently, in a step S214, the EGR solenoid device 12 is forcibly turned off to thereby invalidate positively the exhaust gas recirculation, which is then followed by a step S215 where the intake manifold pressure Pb in the EGR-off state is stored as the EGR-off intake manifold pressure value PbOFF.
In this case, there will equally make appearance a difference between the EGR-off intake manifold pressure value PbOFF and the EGR-on intake manifold pressure value PbON so long as the exhaust gas recirculation control system is operating normally. Accordingly, in a step S216, a pressure difference .DELTA.P between the EGR-on intake manifold pressure value PbON and the EGR-off intake manifold pressure value PbOFF is arithmetically determined in accordance with: EQU .DELTA.P=PbON-PbOFF
In succession, in a step S217, it is decided whether or not the intake manifold pressure difference .DELTA.P mentioned above is greater than a preset reference value FAIL (indicating a lower limit of the intake manifold pressure difference .DELTA.P so long as the normal exhaust gas recirculation is normal). When the result of the decision in the step S217 is affirmative or "YES" (i.e., when .DELTA.P.gtoreq.FAIL), this means that the intake manifold pressure difference .DELTA.P has a normal value (indicating the normal exhaust gas recirculation state). Accordingly, in a step S218, it is decided that the exhaust gas recirculation control system operates normally without suffering any abnormality.
On the other hand, when the decision result of the step S217 indicates that .DELTA.P&lt;FAIL (i.e., when it results in negation "NO"), this means that the intake manifold pressure difference .DELTA.P does not reach the lower limit value of the normal pressure difference range (i.e., exhaust gas recirculation is not effected normally). Accordingly, decision is made in a step S219 that the exhaust gas recirculation control system suffers abnormality.
Parenthetically, when the stable state is decided in the step S211 as mentioned above, this means that the EGR control valve 11 is opened at a predetermined aperture value. Accordingly, the intake manifold pressure Pb in the EGR-validated state (i.e., the EGR-on intake manifold pressure value PbON) is first fetched (steps S212 and S213). Subsequently, the EGR control valve 11 is forcibly closed fully by actuating the EGR solenoid device 12 (i.e., the exhaust gas recirculation is invalidated), whereupon the intake manifold pressure Pb is fetched as the EGR-off intake manifold pressure value (steps S214 and S215).
At this juncture, it should be mentioned that change in the flow rate of the intake air in the engine stable state is smaller than that in the engine deceleration state mentioned previously, because the EGR control valve 11 is fully closed from the state where the EGR control valve 11 is opened at a predetermined value (i.e., from the EGR-on state). This will be explained below.
Let's assume, by way of example, that the EGR ratio (corresponding to the opening degree of the EGR control valve 11) in the stable state is 10% and that the intake manifold pressure Pb in this state is 400 mmHg. Then, the EGR-off intake manifold pressure value PbOFF in the fully closed state can be given as follows: ##EQU1##
Thus, the intake manifold pressure difference .DELTA.P calculated in the step S216 is 40 mmHg (=400-360 mmHg). Consequently, the predetermined value FAIL referenced in the comparison step S217 is set about 20 mmHg so that it can definitely be distinguished from the normal value (40 mmHg) of the intake manifold pressure difference value .DELTA.P.
It should further be mentioned that a processing routine for detecting change or variation of the stable state of the engine 1 is provided, although it is not shown, and activated as an interruption processing periodically at every predetermined time interval for sampling the engine rotation number Ne and the throttle opening degree .theta. for the purpose of detecting change in the stable state on the basis of differences of these parameters before and after the sampling point, respectively. When occurrence of the change in the stable state is detected, the fault detecting routine for the exhaust gas recirculation control system illustrated in FIG. 24 is terminated.
Of course, in the case of the fault detecting routine illustrated in FIG. 24, the occurrence of fault in the exhaust gas recirculation control system can be detected by executing a plurality of processing steps mentioned above by making use of the fact that variation or change in the intake air flow introduced to the engine 1 is reflected onto the intake manifold pressure Pb. Besides, it goes without saying that a processing for turning on an alarm lamp may be performed on the basis of the result of the fault detection processing for informing the driver or operator of the fault event in the exhaust gas recirculation control system.
Next, paying attention to the operation of the bypass air flow rate control means 9, a control operation known heretofore for controlling a bypass air flow rate Qb in the deceleration state of the engine will be described by reference to a timing chart of FIG. 25 which graphically illustrates a relation between a deceleration flag and the bypass air flow rate Qb as well as change of the latter as a function of time lapse. Parenthetically, a broken line in FIG. 25 illustrates a change in the bypass air flow rate Qb in the case of a fourth embodiment of the invention described later on.
At first, it assumed that the engine or motor vehicle is in the running state and that the deceleration flag is set to "0" (indicating that the motor vehicle is not in the deceleration state). In this case, the bypass air flow rate Qb is so controlled as to assume a substantially constant value which is essentially determined by the throttle opening degree .theta.. On the other hand, after the time point t0 at which the deceleration flag is set to "1" (indicating the deceleration state), the bypass air flow rate Qb is arithmetically determined periodically at a predetermined time interval in accordance with the following expression: EQU Qb.sub.n =Qb.sub.n-1 -.beta.
In the above expression, Qb represents a bypass air flow rate at a current time point (hereinafter referred to as the current bypass air flow rate), Qb.sub.n-1 represents a bypass air flow rate at a preceding time point (hereinafter referred to as the preceding bypass air flow rate), and .beta. represents a predetermined value. As can be seen from the above expression, the bypass air flow rate Qb decreases progressively as a function of time lapse in the deceleration state, as illustrated in FIG. 25. Parenthetically, the bypass air flow rate (Qb) decreasing operation mentioned above is generally known as what is called a dashpot operation.
Additionally, it should be mentioned that when an engine load is connected, e.g. upon actuation of the air conditioner, the bypass air flow rate Qb is increased in response to the air-conditioner control signal D. More specifically, the electronic control unit 22 outputs the air-conditioner control signal D to the air-conditioner controller 19A to place the air conditioner in the operating state, while the intake air flow rate is increased to ensure the generation of a demanded output torque by the engine 1.
At this juncture, it is to be recalled that occurrence of a fault in the exhaust gas recirculation control system is decided on the basis of the pressure difference .DELTA.P in the intake manifold pressure Pb between the EGR-off state and the EGR-on state.
Consequently, when the fault decision for the exhaust gas recirculation control system is performed in the deceleration state as described hereinbefore by reference to FIG. 23, the intake manifold pressure difference .DELTA.P as detected may assume different values in dependence on difference of the deceleration state such as difference between a rapid deceleration and a slow deceleration, leading to an erroneous fault detection in the worst case.
Now, possibilities of erroneous fault detection of the fault detecting apparatus for the exhaust gas recirculation control system known heretofore will be explained by reference to FIGS. 26 and FIG. 27, wherein FIG. 26 is a timing chart which illustrates a fault detecting operation in accordance with the procedure shown in FIG. 23 on the assumption that the exhaust gas recirculation control system operates in the normal state without suffering any fault and which shows relations among the EGR flag (indicating the EGR-on and EGR-off state), the engine rotation number (rpm) Ne and the intake manifold pressure Pb and changes thereof as a function of time lapse. Incidentally, in conjunction with the engine rotation number Ne and the intake manifold pressure Pb, single-dotted broken-line curves represent these quantities in a slow deceleration state, while solid-line curves represent them in a rapid deceleration state.
On the other hand, FIG. 27 is a characteristic diagram for illustrating a relation between the engine rotation number Ne (rpm) and the intake manifold pressure Pb (mmHg), wherein a solid-line curve represents the relation in the EGR-off state with a broken-line curve representing the relation in the EGR-on state. In this conjunction, reference character TA (FIG. 26) represents a fault detecting period during which the fault decision enabling conditions are satisfied.
Additionally, a reference symbol a in FIG. 27 indicates a point on the solid-line characteristic curve in the EGR-off state, symbol b indicates a transition point on the broken-line characteristic curve for the EGR-on state from the point a when the change in the engine rotation number Ne is small, and a symbol c indicates a transition point on the EGR-on curve (broken-line curve) when the change in the engine rotation number Ne is large.
In the slow deceleration mode (represented by the single-dotted broken-line curves in FIG. 26), decreasing rate of the engine rotation number Ne is so small that the rate of change in the engine rotation number Ne during the fault detecting period TA can scarcely be observed. In this case, when the exhaust gas recirculation is forcibly put into effect (i.e., validated), transition takes place from the point a on the EGR-off characteristic curve (solid-line curve) to the point b on the EGR-on characteristic curve (broken-line curve), bringing about a remarkable change in the intake manifold pressure Pb, as can be seen from the single-dotted broken-line curve Pb.
When the EGR-on state and the EGR-off state are changed over at time points t1 and t2 during the fault detection period TA (refer to the steps S102 and S104), the EGR-off intake manifold pressure value PbOFF1 and the EGR-on intake manifold pressure value PbON1 in the slow deceleration state can be measured, as can be seen from FIG. 26. In that case, the intake manifold pressure difference .DELTA.P in the slow deceleration state as determined in accordance with the following expression will be greater than the predetermined reference value FAIL mentioned previously, as can be seen from FIGS. 26 and 27. EQU .DELTA.P1=PbON1-PbOFF1&gt;FAIL
Thus, the fault decision means incorporated in the electronic control unit 22 decides in the steps S107 and S108 that the exhaust gas recirculation control system is normal.
On the other hand, in the case of the steep deceleration (refer to the solid-line curves shown in FIG. 26), decreasing in the engine rotation number Ne occurs at a higher rate during the fault detecting period TA, resulting in that the engine rotation number Ne undergoes a significant change with the intake manifold pressure Pb changing rather gently in correspondence to the engine rotation number Ne.
More specifically, referring to FIG. 27, when the exhaust gas recirculation is forcibly validated during the fault detecting period TA, there takes place a transition from the point a on the EGR-off characteristic curve (solid line) to the point c on the EGR-on characteristic curve (broken line). At that time, the intake manifold pressure Pb will of course increase under the effect of the exhaust gas recirculation. However, rate of the change in the intake manifold pressure Pb is relatively small when compared with that in the case of the slow deceleration.
By changing over the exhaust gas recirculation between the off-state (invalidated state) and the on-state (validated state), there can certainly be measured the EGR-off intake manifold pressure value PbOFF2 and the EGR-on intake manifold pressure value PbON2 even in the steep deceleration phase, as illustrated in FIG. 26. In this case, however, the intake manifold pressure difference .DELTA.P2 determined in accordance with the undermentioned expression may assume a value smaller than the predetermined reference value "FAIL", as can be seen from FIGS. 26 and 27. Namely, EQU .DELTA.P2=PbON2-PbOFF2&lt;FAIL
Such being the circumstances, the fault decision means incorporated in the electronic control unit 22 may erroneously decide in the steps S107 and S109 that the exhaust gas recirculation control system suffers abnormality, when the engine is in the steep deceleration state.
Besides, because the relation which the intake manifold pressure Pb bears to the engine rotation number Ne changes when the engine rotation number Ne is approximately 2000 rpm, there exists a possibility of the intake manifold pressure difference .DELTA.P2 increasing.
As the measures for coping with the unwanted situations mentioned above, it is conceivable, by way of example, to inhibit the fault detection processing for the exhaust gas recirculation control system, when the change of the engine rotation number Ne occurs at a high rate (although such measures are not known heretofore). However, if the fault detection is inhibited whenever the change of the engine rotation number Ne is high, the opportunity for the fault detection will be much limited because the deceleration which is not accompanied with change of the engine rotation number Ne can scarcely take place, thus, making it difficult to detect whether the exhaust gas recirculation control system is normal or abnormal. In addition, because the intake manifold pressure difference .DELTA.P assumes different values in dependence on the engine rotation number Ne in the deceleration phase even when the exhaust gas recirculation control system is normal, there may arise the possibility of erroneous detection.
On the other hand, when the fault occurrence detection processing is effected in the stable state as described hereinbefore by reference to FIG. 24, the fault detection processing is inhibited when the change of the throttle opening degree .theta. detected periodically at a predetermined time interval exceeds a predetermined value, because, in that case the engine is regarded as being in the instable state (i.e., the state where the condition for the fault detection is not satisfied). For this reason, there may arise those problems which will be explained below.
FIG. 28 is a timing chart for illustrating a fault detection procedure in the stable state and shows changes in the throttle opening degree .theta., on/off-state of the EGR solenoid device 12, the EGR flow rate and the intake manifold pressure Pb in the stable state of the engine.
As can be seen from FIG. 28, so long as the engine rotation number Ne remains stable, this means that the condition for the fault detection processing is met. Accordingly, after detection of the EGR-on intake manifold pressure value PbON, the exhaust gas recirculation is forcibly invalidated, whereupon the intake manifold pressure Pb is detected to calculate the intake manifold pressure difference .DELTA.P for executing the fault decision.
As first, when the variation or difference .DELTA..theta.a of the throttle opening degree .theta. remains within a predetermined range as encountered during a period from a time point t3 to t4 during which the exhaust gas recirculation is stopped, there can be determined the intake manifold pressure difference .DELTA.P (.DELTA.P=PbON3-PbOFF4 or .DELTA.P=PbON3-PbOFF4a).
However, when a deviation .DELTA..theta. of the throttle opening degree changes beyond the predetermined range at a time point t6 during the fault detecting period TA immediately after a time point t5, the prerequisite condition of the stable state can no more be met, whereby the fault detection processing is inhibited. As a consequence of this, the exhaust gas recirculation will be regained at the time point t6 before the intake manifold pressure Pb can be determined in the EGR-off state.
Subsequently, when the deviation .DELTA..theta. of the throttle opening degree is again stabilized at a value smaller than the predetermined one, the fault detection processing is again started, whereby the exhaust gas recirculation is invalidated during a period from a time point t7 to t8.
To be more concrete, when the deviation .DELTA..theta. of the throttle opening degree increases in the course of execution of the fault detection processing in the stable state, as shown at the time point t6, the fault detection processing (in the EGR-off state) is inhibited on the way, as a result of which decision as to whether or not the exhaust gas recirculation control system is normal is rendered impossible. The fault detection processing is again executed when the deviation .DELTA..theta. of the throttle opening degree becomes stable (period from t7 to t8). In this manner, in dependence on the deviation .DELTA..theta. of the throttle opening degree, the fault detection processing will repetitively be executed, which is unfavorable for the exhaust gas recirculation control.
In other words, there may arise the possibility that the EGR-off state is sustained continuously. To say in another way, the number of times the exhaust gas recirculation is interrupted or stopped during the fault detection period extending, for example, from the time point t5 to t6, is increased, which in turn means that elimination of NO.sub.x -components from the exhaust gas, the intrinsic purpose of the exhaust gas recirculation control, is not satisfactorily performed, involving degradation in the exhaust gas purification performance or capability.
Further, when the change .DELTA..theta. of the throttle opening degree .theta. within a predetermined range takes place in the course of execution of the fault detection processing as encountered during a period from the time point t3 to t4, the EGR-off intake manifold pressure value PbOFF4 (indicated by a point on a solid-line curve in FIG. 28) becomes different from an EGR-off intake manifold pressure value PbOFF4a (indicated by a corresponding point a broken-line curve in FIG. 28), incurring thus an error in the fault detection. Thus, reliability as well as accuracy of the fault detection will be degraded.
Next, problems which may be brought about by the bypass air flow rate control means 9 will be described by reference to FIG. 29 which is a characteristic diagram for illustrating graphically relations between the intake manifold pressure Pb and the engine rotation number Ne by characteristic curves QE (broken line), QoDE (single-dotted broken-line), QEo (solid line), QoE (broken line) and QoEo (solid line), respectively, by using as parameters the bypass air flow rate Qb, the on/off states of the air conditioner and the on/off states of the exhaust gas recirculation.
More specifically, in FIG. 29, the curve QE represents the characteristic between the engine rotation number Ne and the intake manifold pressure Pb when the bypass air flow rate Qb is 200 l/min and when the exhaust gas recirculation is of a normal level .alpha.a, the curve QoDE represents the characteristic when the bypass air flow rate Qb is zero and when the air conditioner is in the off-state with the exhaust gas recirculation being at the normal level .alpha.a, the curve QEo represents the characteristic when the bypass air flow rate Qb is 200 l/min and when exhaust gas recirculation is stopped (off), the curve QoE represents the characteristic when the bypass air flow rate Qb is zero and when the exhaust gas recirculation is at the normal level .alpha.a, and finally the curve QoEo represents the characteristic when the bypass air flow rate Qb is zero and when the exhaust gas recirculation is stopped.
To be more concrete, the solid-line curves QEo and QoEo represent the characteristic relations between the intake manifold pressure Pb and the engine rotation number (rpm) when the exhaust gas recirculation is invalidated (off) in the states where bypass air flow is enabled and disabled, respectively, the broken-line curves QE and QoE represent the characteristic relation when the exhaust gas recirculation is validated in the states where the bypass air flow is enabled and disabled, respectively, and the single-dotted broken-line curve QoDE represents the characteristic when the bypass air flow is zero and when the air conditioner is turned off in the state where the exhaust gas recirculation is validated. Further, reference characters d and f designate points on the characteristic curves QoEo and QEo, respectively, e and g designate points on the characteristic curves QoE and QE, respectively, to which transitions may take place from the points d and the point f upon detection of a fault in the exhaust gas recirculation control system in the deceleration state, as indicated by solid-line arrows, and h designates a point on the characteristic curve QoDE which may transit to the point e as indicated by a broken-line arrow.
As can be seen in FIG. 29, because the intake manifold pressure Pb varies in dependence not only on the exhaust gas recirculation but also on the bypass air flow rate Qb, error may be involved in the EGR-off intake manifold pressure value PbOFF and the EGR-on intake manifold pressure value PbON, if the bypass air flow rate Qb undergoes a change during execution of the fault detection processing, in dependence on whether the exhaust gas recirculation is on or off, incurring erroneous fault detection in the worst case.
More specifically, let's assume that the EGR-off state is changed over to the EGR-on state in the course of the fault detection in the deceleration phase, as illustrated in FIG. 23. Then, there will take place transition from the point d to the point e or from the point f to the point g (as indicated by the solid-line arrow). In that case, the EGR fault detection can be effectuated normally because the bypass air flow rate Qb does not change. However, if the bypass air flow rate Qb changes in the course of transition from the point f to the point e (as indicated by the broken-line arrow), an error will make appearance in the intake manifold pressure difference .DELTA.P. This sort of change in the bypass air flow rate Qb will take place, by way of example, during the dashpot operation in the deceleration phase.
As can be seen from the single-dotted broken-line characteristic curve QoDE shown in FIG. 29, the intake manifold pressure Pb varies in dependence on changes in the engine load such as typified by the on-state or off-state of the air conditioner. Consequently, the intake manifold pressure Pb in the EGR-on state (or EGR-off state) will be accompanied with an error when the engine load (such as the air conditioner) changes in the course of executing the fault detection processing for the exhaust gas recirculation control system, incurring thus as erroneous fault detection in the worst case because of the error in the intake manifold pressure difference .DELTA.P.
In more particular, the relation between the engine rotation number Ne and the intake manifold pressure Pb in the state in which the bypass air flow rate Qb is zero and in which the exhaust gas recirculation is being carried out with the air conditioner put into operation will be such as represented by the characteristic curve QoDE. As can be seen from this characteristic curve, an error is involved in the intake manifold pressure when the engine load changes from the point h on the characteristic curve QoDE to the point e on the characteristic curve QoE, similarly to the case of the change in the bypass air flow rate Qb.
In the foregoing description, although the engine load is assumed to be constituted by the air conditioner, it should be appreciated that the engine load is never restricted to the air conditioner but the phrase is used to encompass other engine load or loads such as a power steering load or the like electric/electronic loads.
Next, problems incurred by change in the atmospheric pressure will be elucidated by reference to FIG. 30. In the exhaust gas recirculation control system known heretofore, the fault detection is made on the assumption that the reference value FAIL is constant independent of the atmospheric pressure. However, there exists a possibility of erroneous fault detection for the exhaust gas recirculation control system, when the atmospheric pressure changes.
FIG. 30 is a characteristic diagram for illustrating a relation between the atmospheric pressure Pa and the intake manifold pressure difference .DELTA.P on the assumption that the engine rotation number Ne is constant (e.g. at 2000 rpm). In the figure, a solid line curve represents the characteristic when the exhaust gas recirculation is normal, while broken-line curves represents the characteristics upon detection of a fault in the exhaust gas recirculation control system, from which it can be seen that the intake manifold pressure difference .DELTA.P changes in dependence on changes in the atmospheric pressure Pa.
When the atmospheric pressure Pa is at 760 mmHg, no problem is incurred in the fault detection performed by comparing the intake manifold pressure difference .DELTA.P with the predetermined reference value FAIL. However, when the atmospheric pressure falls to, for example, 560 mmHg, there arises a possibility of the erroneous detection.
Additionally it should be pointed out that because the intake manifold pressure Pb utilized in the exhaust gas recirculation control mentioned above is not subjected to the filter processing, error is likely to take place in the intake manifold pressure value as detected under the influence of pulsation of the engine operation.
As will now be apparent from the foregoing description, in the case of the fault detection apparatus for the exhaust gas recirculation control system known heretofore, the change in the intake manifold pressure Pb due to the change of the engine rotation number Ne is not taken into consideration when the deceleration state is set as one of the conditions for enabling the fault decision as described previously by reference to the flow chart of FIG. 23. For this reason, there may arise such situation that the intake manifold pressure difference .DELTA.P making appearance between the EGR-on state and the EGR-off state when the conditions for enabling the fault decision are satisfied will become different in dependence on differences in the deceleration such as a rapid or steep deceleration and a gentle or slow deceleration (refer to FIG. 26), giving rise to a problem that erroneous fault detection for the exhaust gas recirculation control system may be incurred in the worst case.
As one of the measures for solving the problem mentioned above, it is conceivable to execute the processing for inhibiting the fault detection procedure when the engine rotation number Ne is abnormally high. In that case, the opportunity for performing the fault detection of the exhaust gas recirculation control system will undesirably be limited, giving rise to another problem. This is because the decision step in which the engine rotation number Ne does not undergo any appreciable change occurs less frequently. Furthermore, because the intake manifold pressure difference .DELTA.P assumes different values in dependence on the engine rotation number Ne in the deceleration state even when the exhaust gas recirculation control system is normal, an erroneous fault detection may be resulted, to inconvenience.
On the other hand, when the stable state is set as the condition to be satisfied in order to enable the fault decision, as described hereinbefore by reference to FIG. 24, change of the throttle opening degree .theta. beyond a predetermined value in the course of the fault detection processing causes the condition for the fault detection to be unsatisfied because then the stable state can no more be ensured, whereby the fault detection processing may be inhibited on the way of execution (refer to the time point t6 in FIG. 28), incurring such unwanted situation that the number of times the fault detection is executed increases, as a result of which the exhaust gas recirculation is stopped frequently, involving degradation in the exhaust gas purification performance of the engine system.
Additionally, since the change of the intake manifold pressure Pb due to the changes in the bypass air flow rate Qb and the engine load (refer to FIG. 29) is not taken into consideration in the fault detection processing known heretofore, there may undesirably arise the erroneous fault detection, because the intake manifold pressure difference .DELTA.P varies when the bypass air flow rate Qb and the engine load change in the course of execution of the fault detection processing.
Finally, because no consideration is paid to the fact that the intake manifold pressure Pb changes in response to the change in the atmospheric pressure Pa, erroneous fault detection may be resulted in case the atmospheric pressure Pa changes in the course of execution of the fault detection processing because the intake manifold pressure difference .DELTA.P will vary correspondingly as a function of the atmospheric pressure.